1. The Field of the Invention
The present invention relates to specific novolak resins made from acetamidophenol monomer. The present invention further relates to radiation-sensitive compositions useful as positive-working photoresists containing these novolak resins.
2. Brief Description of the Art
As integrated circuits (IC) devices become higher in memory density, photolithographic image patterns formed on wafers are required with less than 0.3 micron in resolution. To achieve this goal, photoresists used for photolithographic imaging are required to have higher resolution, faster sensitivity, and more vertical profiles than the state-of-the-art photoresists of a decade ago.
Most common g/i-line positive working-type photoresists currently used in IC device manufacture consist of an alkaline-soluble polymer (e.g., novolak resin) as a binder resin, and a naphthoquinonediazide (DNQ) ester of a polyphenol compound as the photoactive compound. For example, combinations of novolak-type phenol resins and DNQ esters as a positive-working photoresist are disclosed in numerous U.S. Patents. A photoresist composition of cresol formaldehyde novolak and a DNQ ester of trihydroxybenzophenone is also described in "Introduction to Microlithography" edited by L. F. Thompson (ACS Publishing No. 219, p 112-121) as a typical example.
Photoresist performance is influenced by various factors; e.g., structures of the binder resins, photoactive compounds, and other additives, combinations of these components as well as exposure, development and other process parameters. In particular, the structure/property relationships of the binder resins is one of the most complicated subjects.
Binder resins useful for high-performance photoresist formulations, especially for the g/i-line applications are preferably novolak resins which are prepared by addition-condensation reaction of various phenolic derivatives with various aldehydes or ketones in the presence of acids such as oxalic acid, sulfuric acid, and the like. Among various phenolic derivatives previously used as a monomer of the novolak resins, meta-cresol and para-cresol are some of the most useful monomers.
Most novolak resins are made of various combinations of meta-cresol, para-cresol and other co-monomers such as xylenols as disclosed in U.S. Pat. No. 5,652,081 to Tan et al. This patent discloses a positive working photoresist composition comprising an alkali-soluble resin and a 1,2-quinonediazide compound, the alkali-soluble resin being a novolak resin obtained by the condensation reaction of monomers comprising, among other things, p-cresol, m-cresol, and m-acetamidophenol components, as well as several xylenols. The relative amount of p-cresol, m-cresol, and xylenol is from 95 to 55 mol percent, and the relative amount of m-acetamidophenol is from 5-45 mol percent. However, the relative amounts of p-cresol, m-cresol, and m-acetamidophenol disclosed in this patent are 40-50 mol %, 3 mol %, and 10-20 mol % respectively.
Some of the processing condition employed today in the fabrication of semiconductor devices can easily subject the photoresist patterns to high temperatures above their thermal flow points. This results in severe image distortion, loss of yield and high defects which imposes additional restrictions on the process. As the pattern density continues to increase and the printed features continue to shrink below sub-micron scale, narrow trenches and spaces surrounded by large resist patterns become particularly more vulnerable to the resist thermal deformation obliterating such geometry completely. Post development cure treatments of the resist, such as DUV cure or stepped-up post baking cycles are sometimes necessary to apply to improve the resist thermal flow temperature. This requires additional processing steps and the use of specialized equipment. Despite equipment manufacturer's efforts to produce mild temperature plasma reactors, a large portion of the currently used equipment and processes subject the resist to elevated temperatures and harsh conditions. These processes include reactive ion etching, ion milling, metal sputtering and other harsh plasma environments. The necessity of developing high temperature flow resist with adequate processing conditions is therefore unquestionable. This, however, is usually done by compromising much of the resist lithographic properties, such as photospeed and/or resolution. Novolak resins that exhibit high thermal flow temperatures are usually too high in their molecular weight to provide adequate dissolution in the developer, or too brittle to give good resolution and scum free images and adhesion to the silicon, silicon dioxide or particularly to metal substrate. In other cases, where the resist chemistry is designed to cross link the remaining patterned resist after development, it becomes unstrippable and results in scum and undesirable particles contamination to the devices.
This present invention describes a novel class of phenolic resins suitable for formulating high thermal flow temperature positive radiation sensitive resists that also are characterized by good resolution, photospeed and other desirable lithographic properties.